Process of purifying brine used in alkali metal chloride electrolysis



United States Patent 3,250,692 PROCESS OF PURIFYING BRINE USED IN ALKALI METAL CHLORIDE ELECTROLYSIS Karl Hass, Hans Epler, and Gerhard Hauck, Ranzel uher Troisdorf, near Cologne, Germany, assignors to the firm Fel'dmuehle Aktiengeselischaft, Dusseldorf-Oberkassel, Germany, a corporation of Germany No Drawing. Filed Oct. 3, 1962, Ser. No. 228,018 laims priority, application Germany, Oct. 12, 1961,

F 35,120 3 Claims. (Cl. 204-130) The present invention relates to a new and valuable process of purifying alkali metal salt solutions as they are used in alkali metal salt electrolysis.

It is known that, in the electrolysis of alkali metal chlorides such as, for instance, lithium chloride, potassium chloride, sodium chloride, too high a hydrogen concentration in the resulting gaseous chlorine, causes considerable difiiculties. Such generation of hydrogen is usually caused by impurities which are introduced into the electrolytic cell with the starting chlorides. Such impurities decrease the hydrogen over-voltage at the mercury electrode or the amalgam electrode. Especially harmful are compounds of the elements chromium, molybdenum, vanadium, cobalt, germanium, nickel, arsenic, and antimony. These elements will cause a decrease of the hydrogen over-voltage on mercury electrodes in amounts as small as .between 10- and 10- molar concentration.

It is one object of the present invention to overcome these difiiculties and to provide a simple and effective process of purifying the salt solutions before they are used for alkali metal chloride electrolysis whereby substantially all the harmful and the hydrogen over-voltage decreasing cationic impurities are eliminated.

Other objects of the present invention and advantageous features thereof will become apparent as the description proceeds.

In principle the process according to the present invention consists in subjecting the brine, i.e. the alkali metal chloride starting solutions to a preliminary electrolysis on a mercury cathode and an anode, for instance, of amorphous carbon or graphite, thereby operating under a voltage which, on the one hand, is below the deposition voltage of the respective alkali metal ion and which, on the other hand, is high enough to deposit the cationic impurities present in the brine.

Iron is also separated and eliminated by said preliminary electrolysis. Although iron does not disadvantageously affect the alkali metal chloride electrolysis, it may cause, when present in larger amounts, sliming of the amalgam. Therefore, the concomitant removal of iron during the preliminary purification process is another advantage of the present invention.

Furthermore, it is possible to remove other impurities present in the brine by such a preliminary electrolysis, for instance, sodium ions from potassium salt solutions. The amperage or current intensity at the mercury electrode,- when operating under a voltage of 1.7 v. (against an N calomel electrode), amounts to 0.3 ma./sq. cm. or 3 a./sq. m. and at least 0.1 ma./sq. cm. Thus, when subjecting the solution to electrolysis for one hour, there are made available 10,800 coulombs on an area of 1 sq. m. This corresponds to a deposition of 0.1 gram equivalent. When assuming a thickness of the brine layer of 1 m. in this preliminary electrolysis step, about 1190 kg. of brine are present above a mercury surface of 1 sq. m. Assuming that the mean atomic weight of the harmful element is about 50, 0.1 gram equivalent thereof correspond to 5 g. The impurities in the brine are usually present therein in amounts between about 1 part per million and about 5 parts per 3,250,692 Patented May 10, 1966 million. Thus 1190 kg. of brine contain between about 1 g. and about 5 g. of the disturbing elements. It is evident that the above mentioned amperage or current intensity is suflicient to eliminate the disturbing elements from one cu. m. of brine within one hour when using an electrode surface of 1 sq. m.

The following example serves to illustrate the present invention, without, however, limiting the same thereto.

Example A 3 molar potassium chloride solution is electrolyzed in an electrolytic cell on a mercury cathode operating under a voltage of 1.7 v. for one hour. The current density amounts to 0.3 ma./sq. cm. The purified potassium chloride solution, after such a preliminary electrolysis for one hour, does not generate hydrogen when subjected toa control electrolysis on a drop of mercury of a diameter of 1 mm. and with a deposition potential of -2 v. This means that the brine is completely free of harmful elements.

When using a sodium chloride solution in place of said potassium chloride solution, about the same result is achieved; likewise when using a lithium chloride solution.

Of course, many changes and variations in the composition and concentration of the starting salt solutions, in the duration of the preliminary electrolysis step, in the thickness of the layer of the brine, and the like may be made by those skilled in the art in accordance with the principles set forth herein and in the claims annexed hereto.

We claim:

1. In the process of purifying salt solutions used in the alkali metal chloride electrolysis by removal of cationic impurities which are normally present in said salt solution and which decrease the hydrogen overvoltage during electrolysis, the steps which comprise subjecting the starting alkali metal salt solution to a preliminary electrolysis on a mercury cathode and an anode composed of material selected from the group consisting of amorphous carbon and graphite while operating under a voltage below the deposition voltage of the alkali metal ion but sufliciently high to deposit the cationic impurities present in said starting alkali metal salt solution, and continuing said preliminary electrolysis until said impurities have been deposited and substantially completely removed from the salt solution.

2. The process according to claim 1, wherein the voltage under which the electrolytic cell is operated is about 1.7 v. and the current density is about 0.3 ma./sq. cm., and at least 0.1 ma./sq. cm.

3. The method for the purification, prior to electrolysis; of alkali metal chloride solutions from harmful cationic impurities, normally present in said salt solutions in an amount between about one part per million and about 5 parts per million, which method comprises subjecting said salt solution to electrolysis on a mercury cathode and an anode of the group consisting of amorphous carbon and graphite, undera voltage of about 1.7 v. and a current density of about 0.3/ma./sq. cm. until said impurities are substantially completely deposited and removed from the alkali metal chloride solution.

References Cited by the Examiner Angel: I. Electrochemical Soc., volume 102, No. 5, pages 246-251; May 1955.

WINSTON A. DOUGLAS, Primary Examiner. MURRAY TILLMAN, Examiner.

L. G. WISE, H. M. FLOURNOY, Assistant Examiners. 

1. IN THE PROCESS OF PURIFYING SALT SOLUTIONS USED IN THE ALKALI METAL CHLORIDE ELECTROLYSIS BY REMOVAL OF CATIONIC IMPURITIES WHICH ARE NORMALLY PRESENT IN SAID SALT SOLUTION AND WHICH DECREASE THE HYDROGEN OVERVOLTAGE DURING ELECTROLYSIS, THE STEPS WHICH COMPRISE SUBJECTING THE STARTING ALKALI METAL SALT SOLUTION TO A PRELIMINARY ELECTROLYSIS ON A MERCURY CATHODE AND AN ANODE COMPOSED OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF AMORPHOUS CARBON AND GRAPHITE WHILE OPERATING UNDER A VOLTAGE BELOW THE DEPOSITON VLATAGE OF THE ALKALI METAL ION BUT SUFFICIENTLY HIGH TO DEPOSIT THE CATIONIC IMPURITIES PRESENT IN SAID STARTING ALKALI METAL SALT SOLUTION, AND CONTINUING SAID PRELIMINARY ELECTROLYSIS UNTIL SAID IMPURITIES HAVE BEEN DEPOSITED AND SUBSTANTIALLY COMPLETELY REMOVED FROM THE SALT SOLUTION. 